An expert in her field as an investigative curator at the Smithsonian, Dr. Hannah Wood spends her time combing through leaf litter and examining tree trunks in Madagascar, Australia, New Zealand, South Africa and South America in pursuit of tiny and unusual predators. She is investigating members of a small group of unique spiders known as the palpimanoids (Palpimanoidea). These arachnids may be small, (many measuring less than 10 mm in length) but these spiders are the stuff of nightmares for their prey. The highly modified cephalothorax (combination of the head and thorax) and elongated chelicerae (the spider’s jaws) make them excellent predators, well-adapted to their potentially dangerous and fast moving prey. Within the five known extant families of the Palpimanoidea, some are spider specialists, feeding only on other spiders, while some may subsist on small soil-dwelling arthropods, like Collembola. These spiders not only have unusual physical modifications, but also have uncommon methods of hunting, restricted distributions across the globe, low dispersal abilities, and can be traced back millions of years in the fossil record. These qualities make them exciting prey for Dr. Wood, as she strives to learn how these tiny hunters developed their unique characteristics.
Palpimanoids have evolved over millions of years to use very unique methods to catch their prey. The two families of particular intrest to Dr. Wood are the Mecysmaucheniidae and the Archaeidae. In contrast to more typical spiders, these palpimanoids have elongated carapaces (the hardened cover over the cephalothorax) with a modified horizontal chelicerate muscle orientation, which is repsonible for operating their specialized, lengthened and especially mobile chelicerae.
One family that has captured the attention of Dr. Wood is the trap-jaw spiders (Family: Mecysmaucheniidae) which are native only to the cold mountain forests of New Zealand and southern South America. They use their jaws like a bear trap; they hold their long chelicerae out and open, which then snap shut when prey is close enough. Dr. Wood’s dicovery of this unique mechanism is the first of it’s kind among arachnids. The speed at which some of these trap-jaw spiders are able to close their jaws is astonishing, particularly in a New Zealand species, which Dr. Wood has had difficultly observing even using a high-speed camera recording at 40,000 frames per second. Interestingly, this uncommon ability has evolved separately four times within this family. Furthermore, the trap-jaw spiders have developed a mechanism that allows them to store up large amounts of energy, which they then release at once to generate a greater force and speed to snap their chelicerae shut than would be possible with muscles alone. Dr. Wood is using CT scanning technology to get a closer look at the morphological mechanisms behind their incredible speed. In the mysterious forests of Madagascar, Australia and South Africa, Dr. Wood has investigated a close relative to the trap-jaw spider, known as the pelican spider (Family: Archaeidae). Pelican spiders have an even more an extreme carapace modification, which lends them the visual likeness to their namesake, pelicans, but you won’t see these minute predators diving for fish; they are spider specialists. These elaborate morphological adaptations have evolved to compensate for the muscles necessary to work their highly elongated chelicerae, which keep their potentially venomous prey at a distance when they strike. These extraordinary arachnids impale their prey with their venomous fangs positioned at the tip of their chelicerae, until their prey are immobile and no longer a threat, before lowering them to their mouthparts for consumption. Dr. Wood has observed the diversification of these spiders on Madagascar and compared them to other restricted populations in Australia and South Africa to determine the evolutionary pressures acting on their diversification. This one of a kind research is not only interesting, but improves our understanding of evolution and speciation.
About the Authors: Jackie Hoban is a first-year master’s student in Paula Shrewsbury’s lab studying emerald ash borer biological control. Jen Jones is a first-year PhD student in Bill Lamp’s lab studying mosquitoes.
Figure 1 Chelicerae of a trap-jaw spider (Family Mecysmaucheniidae)
Figure 2 An example of the morphological anomaly of the pelican spiders (Family Archaeidae)